Thermal flow meters are known in the art chat control the temperature of liquid flowing through the flow passage to measure a flow rate based on a difference between temperatures of the liquid upstream and downstream of the temperature control part (refer to Japanese Unexamined Patent Application, Publication No. 2006-10322, for example).
Japanese Unexamined Patent Application, Publication No. 2006-10322 discloses a thermal flow meter with a flow passage a wetted portion of which is entirely formed of glass that is obtained by forming a rectangular groove on a glass substrate and bonding another glass substrate with a heat transfer means and a temperature detecting means to the groove side of the earlier glass substrate.